DC to sound

[scarabeus]

I was wondering, when you have a DC power supply (battery), using a single transistor basic amplifier, how will you get sound out of it? My thinking is that the transistor will open up and put the load on the speaker but how do you get both sides of the wave form when you only have a DC supply? I understand if you have two batteries then you can have say +12V 0V -12V ( but then you need two transistors right?). How do you get amplified ac signal from dc power supply with one transistor?

 

[DF96]

Use capacitors.

 

[rayma]

using a single transistor basic amplifier, how will you get sound out of it? how do you get both sides
of the wave form when you only have a DC supply? How do you get amplified ac signal from dc power
supply with one transistor?

A very good question. This is called single supply operation. Input and/or output coupling capacitors
may be involved. The circuit is biased so that at idle, the DC at the output is around half of the
supply voltage, which allows the AC output voltage to go in both directions without clipping off
part of the waveform.
http://www.electronics-tutorials.ws/amplifier/amp_2.html

 

[scarabeus]

Spot on answer rayma, thank you. Another question popped up in my mind. Does that mean that when there is no input signal the transistor is half way between the open and closed state? Also the other day i bought a car amplifier. It says on it 200W. Now i understand it could be 200W musical or pmpo or whatever these people have invented just so they can write bigger and bigger numbers to fool consumers. But if you have a 12v power supply, and a four ohm speaker your max current is 3A which gives you 36W DC power. Does that mean that in amps designed to run on 12V you have to step up the voltage? Which means first you need to convert dc to ac and then step it up?

 

[wg_ski]

First question - yes, half way between open and closed. That's why it's called "analog" as opposed to "digital", which is either on or off.

2nd question - yes, they do step up the 12V to something larger. But they still more often than not inflate the numbers. Good rule of thumb is 50 cents per watt. How much did it cost? Take the number of dollars, multiply by two, and that's pretty close to total wattage of all channels, usualy at 2 ohms (equal to 4 ohms bridged). If you didn't pay $100 for it you probably have less than 200 watts. I'm still running a "1600 watt" Lanzar that cost me $200. It puts out exactly 400 watts bridged at 4 ohms, and has a regulated +/-34V power supply inside.